1. Field of the Invention
The present invention is directed to a double surface coil for a nuclear magnetic resonance apparatus, of the type wherein two individual coils can be optionally connected via a matching network to a transmission/reception circuit, the individual coils having a common junction.
2. Description of the Prior Art
Surface coils are used in magnetic resonance imaging (MRI, or nuclear magnetic resonance imaging, NMR) tomography systems, and in systems for spectroscopic analysis, in which specific parts of an examination subject are to be evaluated with an improved signal-to-noise ratio. Transmission is generally undertaken using a whole-body antenna, and reception is undertaken with the surface coil. A surface coil is disclosed, for example, in European Application 0 280 908, corresponding to U.S. Pat. No. 4,924,868, wherein a double surface coil which is divided into a plurality of separately activatable individual coils, which can be selectively connected to the transmission/reception circuit.
The optimum signal-to-noise ratio is obtained if the surface coil is designed so that its reception range just covers the region of interest of the examination subject. It is preferable for this purpose in specific application to be able to switch the reception range of the surface coil. To this end, the surface coil disclosed in European Application 0 280 908 is in the form of a ladder-shaped structure, by means of which the effective area of the surface coil, as well as the position of the examination region, can be varied. This arrangement is particularly suitable for spinal column diagnostics, because different regions of the spinal column can be successively examined without mechanically displacing the surface coil, by electrically switching the position of the effective area.
Double surface coils are adequate in some applications to achieve the desired three-dimensional selectivity. Such coils are adequate, for example, for mammography, wherein it is desirable to be able optionally image the breasts individually or together. Switching in coil disclosed in the aforementioned European Application 0 280 908 is undertaken by means of varactor diodes, each of which requires a control signal, and the structure also exhibits a high stray capacitance. The structure of European Application 0 280 908 is thus too complicated for use in such double surface coils.